1. Field of the Invention
The present invention relates to an optical pickup, and more particularly, to an optical pickup capable of compatibly adopting disk-type recording media of different formats.
An optical pickup to be adopted in a digital video disk player (DVDP) which can record and reproduce information at a high density should be capable of recording and reproducing on not only a digital video disk (DVD) but also a CD family recording medium such as a compact disk (CD), CD-R (recordable), CD-I, CD-G, etc., in order to achieve compatibility.
However, while the standardized thickness of the existing CD family is 1.2 mm, the standardized thickness of the DVD is 0.6 mm due to an allowable error of the disk inclination, the numerical aperture of an objective lens, etc. Thus, when an optical pickup for a DVD records and reproduces information recorded on a CD, spherical aberrations are generated due to a difference in thicknesses of these media. A sufficient light intensity required for recording an information signal may not be obtained due to the spherical aberrations, or deterioration of a signal during reproduction may occur.
Also, optical sources for reproducing a DVD and a CD have different wavelengths, wherein the wavelength of the existing reproduction optical source for a CD is approximately 780 nm and that for a DVD is approximately 650 nm.
2. Description of the Related Art
A conventional compatible optical pickup capable of recording and reproducing information on disks having different formats as described above includes a single optical source for emitting light having a short wavelength of about 650 nm, suitable for the DVD format, and has an improved structure which can compensate for differences in the thicknesses of disks. As a result, disks of different formats may be interchangeably used in an apparatus adopting such a compatible optical pickup. In this case, deterioration in the intensity of about 5% is generated as compared to an optical pickup adopting an optical source for emitting a 780 nm-wavelength light, but since it is within the allowable error range for reproduction, such a problem is not serious.
However, when the CD-R as one of the CD family is adopted, a difference in sensitivity is generated when an optical source for emitting 650 nm wavelength light is used as compared to when an optical source for emitting 780 nm wavelength light is used. That is, since the CD-R has an organic dye film recording layer, the difference in reflectivity is significant depending on the wavelength of light, as shown in FIG. 1. Thus, when light having a wavelength of about 650 nm is used, the reflectivity is degraded to less than 10%, which makes signal reproduction impossible.
To overcome the above problem, as shown in FIG. 2, a conventional compatible optical pickup having a first optical source 21 for emitting a 635 nm wavelength light and a second optical source 31 for emitting a 780 nm wavelength light has been devised. The first optical source 21 is for a relatively thin disk 10a such as the DVD, and the second optical source 31 is for a relatively thick disk 10b such as the CD.
Light emitted from the first optical source 21 is reflected by a polarizing beam splitter 23, passed through an interference filter 33, a .lambda./4 retardation plate 11 and a hologram 13, and focused on a disk 10 by means of an objective lens 15. The light reflected from the disk 10, passes through the objective lens 15, the hologram 13, the .lambda./4 retardation plate 11, the interference filter 33, the polarizing beam splitter 23, and is incident upon an optical detector 17. The interference filter 33 reflects light emitted from the second optical source 31 through the .lambda./4 retardation plate 11, the hologram 13, and the objective lens 15 to the disk 10 and passes the light reflected by the disk 10 therethrough.
Since the light emitted from the first optical source 21 has a larger diameter than the diffracted pattern formed on the center of the hologram 13, it becomes light which is not diffracted even after it is passed through the hologram 13, that is, 0-order diffracted light, and is focused on the relatively thin disk 10a. On the other hand, the light emitted from the second optical source 31 is diffracted into the +1-order light by a diffracted pattern while it is passed through the hologram 13 and focused on the relatively thick disk 10b.
The compatible optical pickup can utilize the CD-R as a recording medium by employing two optical sources. However, the hologram on which the diffracted pattern is formed is in a position such that the assembly and the optical arrangement are difficult. Also, since a single optical detector is used, precise arrangement of optical axes emitted from two optical sources is needed.